The present invention relates generally to pressurized writing devices and more specifically to pressurized writing devices employing a compressible ink driving member.
Pressurized writing instruments are common to the writing instrument industry and have been in use for many years. Pressurized systems have been used to minimize solvent loss in writing instruments which employ highly volatile solvents and in applications which employ high viscosity inks wherein pressure is needed to force the flow of ink to the writing tip. Moreover, the use of pressurized devices in writing instruments permits the writing instrument to be used for extended periods of time in horizontal and upside-down orientations, and has reduced the need to vigorously shake the instrument to initiate ink flow after storage in an inverted position.
Mechanical and chemical pressurizing devices are two types of pressurizing systems which have been employed in writing instruments. Mechanical pressurizing devices contain a mechanism, such as a spring, to maintain constant pressure on the writing medium as the writing medium is consumed. Gas-pressurized systems typically use a pressurized gas, such as nitrogen, to feed ink to the point or nib of the writing instrument. Some of these devices produce a pressurized gas, such as nitrogen, through chemical reactions, fermentation, and the like. The gas maintains pressure on the writing medium for continuous supply of the medium to the point or nib of the writing instrument. U.S. Pat. No. 3,130,711 to Eckerle describes writing instruments employing pressurized gas systems. Examples of commercially available writing instruments which employ pressurized gas systems include the Papermate Erasermate2(trademark), produced by The Gillette Company (USA), of Boston, Mass., and the Fisher Space Pen(copyright), produced by the Fisher Space Pen Company of Boulder City, Nev.
The loss of gas through leakage or permeation is a major concern in pressurized gas systems. Thus, maintenance of an airtight seal and the prevention of gas leakage from the seal are important factors in designing gas-pressurization systems. Often, the gas retention properties of the writing instrument are the limiting factor determining the usable shelf life of the product. Maintenance of an airtight seal is difficult to accomplish in gas-pressurized systems for various reasons. Certain gas systems do not permit the use of certain bonding techniques due to the potential interaction or exposure of the bonding material or solvent with the pressurizing gas. In an effort to provide more effective sealing, liquid sealants have been used in conjunction with plug members to maintain the integrity of the gas seal. However, this combination often produces internal pressure variations which interfere with the uniform flow of the writing medium. The liquid sealant may also be chemically incompatible with at least one of the materials of the writing instrument, causing those materials to interact.
An additional design consideration with respect to gas-pressurization systems is the interaction of the component containing the gas pressurization system (hereinafter the xe2x80x9cink tubexe2x80x9d for the sake of simplicity) with the writing medium as well as the stability of the ink tube. If a volatile writing medium is used, the ink tube must be made of a material that is not gas permeable moreover, the ink tube must be able to withstand stresses during use, such as imparted by the pressurization system. Any crack in such component will allow the gas to escape, eliminating the pressure required to feed the ink to the writing tip and thus rendering the writing instrument unusable. For this reason, nylon is often used. However, nylon is more expensive than some alternative materials. Moreover, since Nylon is relatively weak, a nylon ink tube must have thick walls to withstand stresses occurring during normal use. The formation of a nylon tube with sufficiently thick walls increases material costs of an already relatively expensive ink tube. Nylon also has shrinkage and creep properties inferior to alternative materials, resulting in relatively less dimensional stability. Furthermore, nylon is somewhat hygroscopic, and generally must be dried before being molded to form the ink tube to avoid dimensional instabilities which may result from water absorption.
Another design consideration with respect to gas-pressurized systems is the shape and dimension of the ink tube. In particular, the ink tube must have a relatively small cross-sectional area to ensure that the meniscus formed at the top of the ink supply prevents the pressurizing gas from flowing to the writing end of the writing instrument in the event the writing instrument is placed on its side, or turned upside-down. Transfer of the pressurizing gas to the writing end of the writing instrument raises the possibility of a gas pressure leak. Particularly, such transfer may result in the creation of trapped pressurized gas bubbles within the ink supply when the writing instrument is placed in position for use. If such bubbles reach the writing ball, the gas will escape. Once insufficient gas remains in the writing instrument to force the ink to the writing tip, the writing instrument will be rendered unusable.
A mechanical pressurization system has several advantages over a gas pressurized system. These advantages include simplification of the assembly process, greater control and regularity of the ink pressure, no risk of loss of pressurizing gas to the atmosphere, and no risk of interaction of the pressurizing gas with the components of the writing instrument. However, prior attempts to create a mechanically pressurized writing instrument have also encountered disadvantages, such as: operation being reasonable only in the vertical position (such as in U.S. Pat. No. 4,937,594 to Niemeyer), uneven force being applied to the ink column as the ink supply is exhausted, and added expense and complexity due to the precision required in producing an ink driving member which will fit tightly enough within the ink tube to prevent leakage past the ink driving member yet will still slide freely within the ink tube. Usually, the addition of a lubricant is required to prevent any leakage past the piston and to facilitate movement. U.S. Pat. No. 3,282,255 to Killen uses a mechanical manual xe2x80x9cpumpxe2x80x9d to pressurize the ink in the reservoir but cannot provide a consistent force on the ink reservoir.
Manufacturing considerations with respect to the ink tube may also complicate design of a pressurization system which functions as desired. For instance, to facilitate manufacturing, an injection molded ink tube typically is tapered towards the writing tip to facilitate separation from the mold pin therein upon completion of the molding process. The unyielding, incompressible piston members of prior art pressurization systems cannot maintain a seal at the wider section of the ink tube yet also fit in the narrowest section to expel all ink from the writing instrument.
There remains a need to provide an improved pressurized writing instrument in which the internal pressure is maintained throughout the entire life of the writing device, without overly complex or expensive seals, to allow for smooth and continuous flow of ink, even where the diameter or dimensions of the ink tube vary, and regardless of the orientation of the writing device.
In accordance with the principles of the present invention, a writing instrument employing a pressurizing system to feed ink to a writing tip is provided with a compressible ink driving member. The compressible ink driving member is capable of deforming to conform to the contours of the walls of the ink tube. The compressible ink driving member preferably acts as a fluid seal to prevent the writing medium from flowing past the ink driving member. Moreover, the deformed ink driving member facilitates wiping of the writing medium from the ink tube walls.
The compressibility of the piston member also has advantages in the manufacture and assembly process of the writing instrument by eliminating the need to manufacture precision parts with tight tolerances. Any minor variations in the size of the piston member or the ink reservoir is compensated for by the ability of the piston member to deform.